In most four-wheel steer vehicles, the front wheels are steered together in direct and usually linear relation to the steering input of the driver, as in a two-wheel steer vehicle. The rear wheels are steered together as a function of the front wheel steering angle and/or the vehicle speed, to provide desired ride or handling characteristics. The desired ride or handling characteristics are electrically or mechanically simulated to define a reference model, and the rear steering angle required to achieve the desired characteristics is carried out by the rear wheel steering mechanism.
An early mechanization of a four-wheel steer control, as generally set forth above, was described in detail in a paper presented by K. J. McKenna at the Joint Automatic Control Conference held at University of Texas in June, 1974, entitled "A Variable Response Vehicle--Description and Applications". The control described in the paper incorporated a reference steer mode in which a reference model supplied the desired yaw rate and lateral velocity, and a closed-loop control developed the steering commands in relation to a comparison between the desired and actual (measured) parameters. Being closed-loop, this type of control responds not only to driver steering inputs, but also to external disturbances which affect the measured parameters.
A control dilemma encountered in a closed-loop four-wheel steer system of the type generally referred to above, concerns rationalizing the control response to driver inputs and externally generated inputs. To be able to stabilize the vehicle in response to externally generated inputs--wind gusts, for example--the yaw response of the rear steering control must be relatively fast. On the other hand, such relatively fast rear steering yaw response tends to produce a sudden reaction to driver steering inputs, which may be undesirable at relatively high vehicle speeds. Moreover, control responses to noise in the signal representing the driver steering input also tends to degrade drivability.